We have previously reported that one of the genes that is rapidly induced in mouse 3T3 cells by serum growth factors (jun-B) encodes a protein related to the onco-protein v-jun. By using jun-B as a probe, we have isolated a cDNA encoding a second member of the jun family (jun-A) that is the murine version of the protooncogene c-jun, which encodes the mammalian transcription factor AP-1. jun-B and jun-A (c-jun) have two highly conserved regions and two regions with little sequence similarity. Like jun-B, jun-A (c-jun) is rapidly activated by serum, platelet-derived growth factor, or fibroblast growth factor and is superinduced by serum in the presence of an inhibitor of protein synthesis. Both jun proteins are likely to play a role in regulating the genetic program induced by growth factors.